A step-ratio automatic transmission uses multiple friction elements for automatic gear ratio shifting. A ratio change from a low gear ratio to a high gear ratio occurs in a synchronous clutch-to-clutch upshift as one friction element is engaged and a second friction element is disengaged. One friction element may be referred to as an off-going clutch (OGC). It is released while a second friction element, which may be referred to as an oncoming clutch (OCC), engages to create the upshift. The upshift event is divided into a preparatory phase, a torque phase and an inertia phase. During the preparatory phase, the OCC actuator is stroked to prepare for its engagement, while the OGC torque-holding capacity is reduced to prepare for its release. During the torque phase, the OCC torque is raised in a controlled manner while the OGC is still engaged or allowed to slip at a controlled slip rate.
Simultaneous engagement of the OCC and release of OGC in a conventional transmission upshift may result in a momentary activation of two torque flow paths through the gearing. During the torque phase, the lower gear speed ratio from input to output is maintained. However, as the OCC gains torque capacity and the OGC loses it, more of the input torque is routed through higher gear path until, when the OGC no longer has any capacity, all of the torque is routed through the higher gear path, which has a lower torque ratio. Thus, in the small timespan of the torque transfer, the input torque goes from being multiplied by a higher amount to a lower amount before the inertia of the subsequent speed change raises the output torque again. This momentary dropping and subsequent rise of output torque is known as the “torque hole.” This is perceived by a vehicle occupant as an unpleasant shift shock. The inertia phase begins when the OGC is released or has no significant torque capacity.